The present invention relates to a vulcanizing method of a pneumatic bias tire, in which a shape of a rubber bladder for vulcanization is improved. More particularly, the present invention relates to a method optimal for vulcanizing a pneumatic bias tire having an aspect ratio of 25% or more and 55% or less.
FIG. 7 is a view showing an example of the conventional vulcanizer for a pneumatic bias tire. As shown in FIG. 7, in the vulcanizer, an unvulcanized tire (green tire) 2 is inserted between upper and lower molds 3 and 4, expanded from the inside thereof by means of a rubber-made bladder 11, and pressed to the inner surfaces of the metallic molds 3 and 4 with a high pressure. Thus, vulcanization is performed for the unvulcanized tire 2. On the other hand, the rubber-made bladder 11 has a shape as shown in FIG. 8, when it is not expanded. In the vulcanizing method of a pneumatic bias tire using the tire vulcanizer as described above, it is important to close the molds after the unvulcanized tire 2 made to be cylindrical is expanded to a shape similar to profiles of the metallic molds by means of the expansion of the bladder 11, from the viewpoint of improving a non-defective ratio.
Heretofore, in the case of designing a bladder used for the tire vulcanizer, in order not to lower the number of vulcanizing steps that a single bladder can perform, that is, in order to maintain the bladder life for a long period of time, a value is set in a range approximately from 1.1 to 1.6, which is obtained by dividing a tire inner surface diameter A of an objective pneumatic bias tire by a bladder outer diameter B when the bladder is not expanded. The bladder outer diameter B relative to the tire inner surface diameter A is set in the above-described range, and the inside of the bladder is filled with a heating medium for vulcanization. Thus, as shown in FIG. 9, the bladder is expanded to a substantially spherical shape.
In the case where a pneumatic bias tire having an aspect ratio of 60% or more is vulcanized by using the bladder as described above (FIG. 7), the profiles of the metallic molds and the shape of the bladder when it is expanded are approximately coincident with each other, thus enabling vulcanization with hardly causing a vulcanization defect and the like. However, in the case where vulcanization is performed for a pneumatic bias tire having an aspect ratio of 25% or more and 55% or less (FIG. 10), when the bladder 11 is expanded to a spherical shape as described above, in a state where the molds are open, a tread portion of the unvulcanized tire 2 exceeds a portion of the metallic molds for vulcanization which corresponds to a portion of the tire diameter, as shown by dotted lines in FIG. 10. The portion of the unvulcanized tire 2 exceeding as described above may be nipped between the upper and lower molds 3 and 4, thus tending to cause defects.
For this reason, in vulcanizing the pneumatic bias tire having an aspect ratio of 25% or more and 55% or less, the unvulcanized tire is expanded to only a little extent by means of the bladder, and the molds are closed while adjustment is performed so that the outer diameter of the unvulcanized tire cannot exceed the dimension of the metallic molds which corresponds to the tire outer diameter. However, in this case, since a tire side portion is not sufficiently expanded, a vulcanization defect has tended to occur in the tire side portion. Moreover, since an angle change of carcass cords does not become uniform, there has existed a problem that a great variation tends to occur in an outer diameter dimension of a product tire. Furthermore, since a transformation amount of the tire side portion inside the metallic molds is needed to be large, there has existed a problem that a bladder life is shortened due to a friction between the unvulcanized tire and the bladder.
Moreover, in the bladder for the tire vulcanizer, which is described above, for the purpose of improving a rubber flow in the metallic molds and of reducing an occurrence of vulcanization defects and a variation of the tire outer diameter dimension, a large number of vent grooves are arranged on the surface of the bladder, and air remaining between the unvulcanized tire and the surfaces of the metallic molds is removed through these vent grooves, as described in Japanese Patent application Kokai publication No. 6-143288. Specifically, as shown in FIG. 8, a large number of vent grooves 15 inclining relative to the tire diameter direction and extending linearly are arranged in parallel with each other on the surface of the bladder 11.
However, when the vent grooves 15 as described above are arranged on the surface of the bladder 11, the vent grooves 15 and the carcass cords interfere with each other, thus causing inconveniences such as a disorder of an end count of the carcass cords and an exposure of the carcass cords to the inner surface of the tire, which is caused by a phenomenon that the rubber of the carcass cords are drawn by the vent grooves 15. In particular, in a pneumatic bias tire for competition having a rim diameter of 4 to 6 inches and an extremely thin inner liner rubber, since a flow of the rubber of the carcass cords into the vent grooves is unavoidable, an inner liner gauge has had to be thicker than necessary. Moreover, when the vent grooves as described above are arranged on the surface of the bladder, the bladder life has tended to be shortened due to the friction between the unvulcanized tire and the bladder.
The object of the present invention is, in vulcanizing a pneumatic bias tire having an aspect ratio of 25% or more and 55% or less, to provide a vulcanizing method of the pneumatic bias tire, which is capable of reducing an occurrence of vulcanization defects and a variation of a diameter dimension, in which a nip of a tread portion of an unvulcanized tire into metallic molds is avoided, and a bladder life is not shortened.
Another object of the present invention is to provide a vulcanizing method of a pneumatic bias tire, which is capable of further reducing the occurrence of the vulcanization defects and the variation of the diameter dimension, in which a disorder of an end count of carcass cords per unit width and an exposure of the carcass cords to the inner surface of the tire are prevented, and air remaining between the unvulcanized tire and the inner surfaces of the metallic molds are removed well.
In order to achieve the foregoing object, the vulcanizing method of a pneumatic bias tire of the present invention, comprises the steps of: expanding a rubber-made bladder from the inside of the unvulcanized tire in the metallic molds; and vulcanizing the pneumatic bias tire having an aspect ratio Y of 25% or more and 55% or less from the unvulcanized tire, in which a bladder stretch ratio X of an outer diameter of the bladder when it is not expanded relative to a tire inner surface diameter, when the unvulcanized tire is pressed to the inner surfaces of the metallic molds by means of expansion of the bladder, is set in a range expressed by the following expression:
X=xcex2xe2x88x92Y/xcex1,
where 0.37 less than xcex1 less than 0.42, 310 less than xcex2 less than 320.
In the case where a relation is made between the bladder stretch ratio X and the aspect ratio of the tire Y as described above, in vulcanizing the pneumatic bias tire having an aspect ratio of 25% or more and 55% or less, the bladder follows the profile, particularly the shape of the side portion of the tire in the metallic molds in a state where the bladder is sufficiently expanded. Thus, a nip of a tread portion of an unvulcanized tire into the metallic molds and shortening of the bladder life due to a friction between the unvulcanized tire and the bladder are avoided. Accordingly, the occurrence of the vulcanization defects and the variation of the outer diameter dimension can be reduced.
In the present invention, it is preferable that vent grooves are arranged in an area where the surface of the bladder contacts at least a tire side portion, and a pattern of the vent grooves is made to be a mesh shape of a polygon (preferably, pentagon to dodecagon). By arranging the vent grooves having the polygonal mesh shape pattern as described above, air remaining between the unvulcanized tire and the inner surfaces of the metallic molds can be removed well, thus further reducing the occurrence of the vulcanization defects and the variation of the outer diameter. These vent grooves having the polygonal mesh shape pattern are hard to interfere with the carcass cords, thus preventing a disorder of an end count of the carcass cords and an exposure of the carcass cords. Moreover, even if the bladder stretch ratio X becomes larger, the vent grooves having the polygonal mesh shape pattern disperse a tensile strength uniformly, thus preventing a significant shortening of the bladder life.
Preferably, a groove width of the vent groove is set in a range from 2 to 5 mm, and a groove depth thereof is set in a range from 0.2 to 0.7 mm. By setting such dimensions, vulcanization can be performed even for the pneumatic tire having an extremely thin inner liner at a relatively low defect ratio.
As described above, with the vulcanizing method of a pneumatic bias tire of the present invention, a pneumatic bias tire for competition can be provided, which has an aspect ratio of 25% or more and 55% or less, and has preferably an extremely thin inner liner and a rim diameter of 4 to 6 inches, in which vulcanization defects and the like are very little and yield during vulcanization is good.